1. Field of the Invention
This invention relates generally to level detecting circuits and, more particularly, is directed to a level detecting circuit of the logarithmic compression type.
2. Description of the Prior Art
Noise reduction circuits for reducing noise and distortion which accompany a reproduced information signal are well-known in the art. Such noise reduction circuits are designed to increase the dynamic range of the signal that can be recorded and reproduced from a recording medium such as a magnetic tape. Such noise reduction circuits generally incorporate an encoding process which compresses the level of the information signal prior to recording the signal on the recording medium, and a decoding process which expands the level of the information signal, during the reproducing operation, with a characteristic which is complementary to the compression characteristic. As a result, various restrictions imposed on the dynamic range of the information signal by the signal transmission paths and the recording medium can be eliminated.
One such noise reduction circuit uses a transmission circuit having a variable compression/expansion characteristic which is dependent on the level and/or frequency of the input information signal. Such transmission circuit has a gain controlled amplifier, such as a voltage controlled amplifier, which effects the aforementioned compression and expansion operations, and a level detecting circuit which supplies a control voltage corresponding to the input information signal to the voltage controlled amplifier for controlling the variable compression/expansion characteristic.
However, voltage controlled amplifiers and level detecting circuits having greater precision and being adapted to greatly increase the dynamic range of the information signal have recently been required in audio tape recorders for greatly reducing noise accompanying the information signal. It is also desirable that such circuits overcome various problems with the information signal caused by noise modulation and overshoots in the signal. Accordingly, where it is required that the dynamic range used with a noise reduction circuit exceeds 60 dB, it is desirable to use an exponential-to-logarithmic conversion circuit or logarithmic compression circuit for the level detecting circuit which produces the control voltage. An effective value level detecting circuit of the logarithmic compression type is disclosed in U.S. Pat. No. 3,681,618.
When the level of the input information signal is abruptly increased, the resulting reproduced output signal has a corresponding overshoot portion which is substantially greater than the desired level of the output signal. The time within which this overshoot portion falls back to its desired level is termed the attack time or rise time constant. However, it becomes difficult to choose a correct attack time since an attack time which is too long will distort the sound which is eventually reproduced and an attack time that is too short will result in a clicking noise in the reproduced sound. An optimum attack time is therefore set in the range of approximately 100 .mu.sec. to 10 msec. In like manner, when the input signal level falls from a high value to a low value, a negative overshoot occurs and the time within which the level of the signal returns from the overshoot level to its desired level is termed the recovery time or fall time constant. The recovery time is optimally set for a comparitively long time, for example, in the range from several ten msec. to several hundred msec., that is, at least one hundred times the attack time. However, when the aforementioned effective value detecting circuit is used in a noise reduction circuit, the maximum ratio that can practically be obtained between the recovery time and attack time is approximately equal to 4. In such case, with a rapidly rising input signal, it is extremely likely that overshoots will be produced, thereby causing saturation of the recording medium and a consequent deterioration of the reproduced signal.
Further, it may be desirable to use different rise time constants for different input signals. In particular, it is desirable to use a relatively small or fast rise time constant for sharply rising high frequency, high level input signals so as to avoid the problem of deterioration in the quality of the reproduced sound due to encoder overshoot. In other words, since the encoding portion of the noise reduction circuit generally includes a limiter circuit for clipping the encoded signal for levels greater than a predetermined level, with a small rise time constant, clipping of the encoded signal may not be necessary so that only minimal saturation of the recording medium occurs, thereby preventing deterioration in the quality of the reproduced sound. On the other hand, it is also desirable to use a relatively large or slow rise time constant for low and intermediate signals and for high frequency, low level input signals to prevent deterioration of the reproduced sound quality due to phase distortion and the like, and to prevent adverse effects from noise in pulse form. It is therefore desirable to provide a level detecting circuit in which the rise time constant varies in accordance with the input signal in satisfaction of the aforementioned incompatible requirements.